[erlang-questions] Why do we need modules at all?

Joe Armstrong <>
Tue May 24 13:25:52 CEST 2011


On Tue, May 24, 2011 at 11:46 AM, Jesper Louis Andersen <
> wrote:

> On Tue, May 24, 2011 at 11:05, Kresten Krab Thorup <>
> wrote:
> >
> > One of the things he's arguing is that modules should be VALUES.  A
> "module" in newspeak is kind of a template (function), which take other
> module instances as arguments.  So you hook up a system by applying such
> templates (possibly using letrec).
> >
>
> This is a great idea. Why? Because it is the path of existential types :-)
>
> Notice that in Erlang, a module is a Value. It is possible to call
> Mod:Fun(..). Second, notice that the parameterized module extension
> allows us to parameterize a module over some other module.
> Specifically we could have
>
> -module(RBTree, [OrderMod]).
>
> where OrderMod is a module which has a function
>
> -opaque t() :: .....
> -export_type([t/0]).
> -spec order(t(), t()) -> lt | eq | gt.
>
> and so we have generalized our RedBlack trees to be able to use any
> ordering relation, not simply the one that is built it.
>
> The style Gilad is advocating is the "Fully functorial style" from the
> Standard ML community. A Functor[1] in SML terminology is a function
> from module to module ("functor" stems from category theory where it
> designates a "function"[2] one level up). In the style, you build up
> programs by stitching together smaller modularized parts building
> larger and larger modules in the process.
>
> Example: The gen_server is a functor from a module (which obeys the
> gen_server behaviour and has handle_* functions) to a specific
> instance of a server.
>
> It also gives you quite the path towards the concept of OOP, as can be
> observed from the parameterized module extension and its uses in
> various places.
>
> In languages with static types, you can of course enforce that functor
> applications are type-correct. If you allow modules as values in such
> languages you get existential types which is a way to get something
> OOP-like into those languages[3].
>
> So after a little tour around: Erlang already has this :P
>
> Modules are an essential part of building large systems. The very
> notion that you can pack up functions in a module and ship them as a
> unit is quite important. Also the notion you can replace a
> module-component at once. I feel that the problem of function
> placement in the right module is less of a problem compared to the
> hell it would be with one large global namespace. And it is much more
> than just a "bunch of functions thrown together".
>
> Specifically, modules allows me to have an *abstract* view of a data
> structure. In the above, I have no idea what t() is. I am only allowed
> to compare two t()'s to each other with OrderMod:compare(T1, T2). That
> is, the only algebraic operation I am allowed to do is this. Other
> parts of the program may have a different view of t() and be able to
> do more with it, but to me, it is an opaque term. It matters because
> someone can go replace t(), and my code does not have to be changed at
> all. This is impossible if t() leaks. And I am grateful the dialyzer
> can find places where it is leaking.
>
> Likewise, a module can hide the fact it is implemented with a
> gen_server. So I can go replace it with a gen_fsm, and nobody has to
> alter their code because the API i exported was exactly the API I will
> keep stable.
>
> Processes componentize the heap.
> Modules componentize the code.
>
>
If we think of modules as containers of functions, then a module is a "set
of functions" -
my problem is with granularity. The smallest unit of reuse is the module.
Suppose my application
want just one function from a particular module - I'm forced to load the
entire module.

This hampers reuse - I *often* read library code and find a single function
and cut-and-paste it into
my new code. It also discourages sharing - since the smallest unit I can
share is a module.

The unit of distribution is an application - a set of modules - why this
could not be a list of functions
I do not know.

I think I'd like to view an application as something that offered a message
based service that internally
was constructed from a set of functions rather than a set of modules.

 /Joe



>
>
> [1] Beware of connotating the word functor in programming with
> something specific in computer science. The meaning is different in
> Standard ML, in Haskell and in C++.
>
> [2] I am lying. The basic notion is a morphism of which the function
> is a special case when considering the category of Sets.
>
> [3] I still don't get why you can't use static typing in Erlang. The
> conservative approach is simply to use polymorphic variants for all
> message passing and it would essentially work right out of the bat.
>
> --
> J.
>
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